Students' understanding of physics concepts can now easily be probed with computer animations that allow motion to be viewed dynamically. But when and how should these questions be used? Most research in the area of animation has focused on the effect of animation on students' reading comprehension and in "learning-by-doing" interactive activities. There is little previous research on the effect of animation on students' performance on problem solving activities or conceptual reasoning questions . In an effort to address this lack of information, Physlets have been used to investigate how animation influences the way students answer conceptual questions such as those found on the Force Concept Inventory (FCI) .